This invention relates in general to flexible belts, and more specifically, to flexible electrostatographic imaging belts having a skewed seam configuration and methods for fabricating the flexible electrostatographic imaging belts.
Flexible belts, such as electrostatographic imaging members, are well known. Typical electrostatographic flexible imaging members include, for example, photoreceptors for electrophotographic imaging systems, and electroreceptors or ionographic imaging members for electrographic imaging systems. Electrostatographic imaging member belts typically have a welded seam. For electrophotographic applications, the imaging members preferably comprise a flexible substrate coated with one or more layers of photoconductive material. The substrates are usually organic materials such as a film forming polymer. The photoconductive coatings applied to these substrates may comprise inorganic materials such as selenium or selenium alloys, organic materials, or combinations of organic and inorganic materials. The organic photoconductive layers may comprise, for example, a single binder layer or multilayers comprising, for example, a charge generating layer and a charge transport layer. The charge generating layer is capable of photogenerating holes and injecting the photogenerated holes into the charge transport layer. The basic process for using electrostatographic flexible imaging members is well known in the art.
As more advanced, higher speed electrophotographic copiers, duplicators and printers were developed, degradation of image quality was encountered during extended cycling. Moreover, complex, highly sophisticated duplicating and printing systems operating at very high speeds have placed stringent requirements including narrow operating limits on photoreceptors. For example, the numerous layers found in many modern photoconductive imaging members must be highly flexible, adhere well to adjacent layers, and exhibit predictable electrical characteristics within narrow operating limits to provide excellent toner images over many thousands of cycles. One typical type of multilayered imaging member that has been employed as a belt in electrophotographic imaging systems is a photoreceptor comprising a supporting substrate, a conductive layer, a hole blocking layer, an adhesive layer, a charge generating layer, a charge transport layer, a conductive ground strip layer adjacent to one edge of the imaging layers, and an anti-curl back coating at the back side of the supporting substrate opposite to the electrophotographic active layers. This imaging member may also comprise additional layers, such as an anti-curl back coating layer to render the desirable imaging member flat and an optional overcoating layer to protect the exposed charge transport layer from wear.
The electrophotographic imaging flexible member is usually fabricated from a sheet cut from a web. The sheets are generally rectangular in shape. All sides may be of the same length, or one pair of parallel sides may be longer than the other pair of parallel sides. The sheets are fabricated into a belt by overlap joining the opposite marginal end regions of the sheet. A seam is typically produced in the overlapping marginal end regions at the point of joining. Joining may be effected by any suitable means. Typical joining techniques include welding (including ultrasonic), gluing, taping, pressure heat fusing and the like. Ultrasonic welding is generally the preferred method of joining for flexible polymeric sheets because of its speed, cleanliness (no solvents) and production of a thin and narrow seam. In addition, ultrasonic welding is preferred because it causes the generation of heat at the contiguous overlapping marginal end regions of the sheet to maximize melting of one or more coating layers therein and facilitate direct contact of the supporting substrate for fusing at the overlapping end regions.
The flexible imaging member used in an electrophotographic imaging machine can be a typical photoreceptor belt fabricated by the ultrasonic welding of the overlapped opposite ends of a rectangular sheet. The expression "rectangular", as employed herein, is intended to include four sided sheets where the length of two parallel sides is unequal to the other two parallel sides and the adjacent sides are perpendicular to each other, or sheets where all the sides are of equal length to form a square. In the ultrasonic seam welding process, ultrasonic energy transmitted to the overlap region is used to melt the coating layers of the photoconductive sheet thereby providing direct substrate fusing into a seam. The ultrasonic welded seams of multilayered photoreceptor belts are relatively brittle and low in elasticity and toughness. This joining technique by ultrasonic welding process, can result in the formation of two flashings and two splashings that project, respectively, beyond the edges of the photoreceptor belt and either side of the overlap region of the seam. The seam flashings can be removed from either edge of the belt with the use of a reciprocating punch or notching device according to the procedures described in U.S. Pat. No. 4,937,117, the entire disclosure thereof being incorporated herein by reference. The reciprocating punch has small circular section and removes the flashings and part of the seam to form a generally semi-circular notch in either edge of the belt. However, with the overlap and presence of two seam splashings, the top splashing over the charge transport layer while the bottom one on the anti-curl back coating, the flexible imaging member is about 1.6 times thicker in the seam region than that of the remainder thereof (in a typical example, 188 micrometers versus 116 micrometers).
The photoreceptor belt undergoes stress/strain as it is cycled over a plurality of belt support rollers in an electrophotographic imaging apparatus. The excessive thickness of the photoreceptor belt in the seam region due to the presence of the splashing and seam overlap results in a large induced bending strain as the seam passes over each support roller. Generally, small diameter support rollers, highly desirable for simple, reliable copy paper self-stripping systems, are used in compact electrophotographic imaging apparatus which require a photoreceptor belt to operate in a very confined space. Unfortunately, small diameter rollers, e.g., less than about 0.75 inch (19 millimeters) in diameter, raise the threshold of the mechanical performance criteria for photoreceptor belts to such a high level that premature photoreceptor belt seam failure can frequently occur thereby shortening the service life of the belt. For example, when bent over a 19 millimeter roller, a conventional commercially available XEROX.RTM. welded photoreceptor belt seam may develop a 0.96 percent induced bending strain. Compared to a 0.59 percent tensile bending strain for the rest of the belt, the 0.96 percent tensile strain in the seam region of the belt represents a 63 percent increase in stress placed upon the overlapped seam and splashing which leads to the development of seam cracking, delamination, and tearing during extended cycling.
Under dynamic fatiguing conditions, the seam overlap and splashing provide a focal point for stress concentration and become the initial point of failure that affects the mechanical integrity of the belt. The excessive thickness of the seam overlap and splashing, thus tend to shorten the mechanical life of the seam and, hence, adversely affects the service life of the flexible member belt in copiers, duplicators, and printers. The excessive seam thickness and rough topography of the outer splashing cause the development of large lateral friction forces against cleaning blades during electrophotographic imaging and cleaning processes. Moreover, the collision of the inner seam splashing with toner image acoustic transfer assist devices and drive rolls also disturb the belt during cycling, resulting in significant belt cycling and transporting speed changes. These mechanical interactions have been observed to severely affect the life of the imaging belt, exacerbate blade wear, and induce belt velocity variations during imaging belt machine function.